1. Field of Invention
This invention relates to a noise canceling circuit for canceling noise included in alternating (AC) current supplied from an AC power supply in various kinds of electric appliances for household use and office-automation use, such as a sewing machine, facsimile machine or a printer.
2. Description of Related Art
Generally, in electric appliances for household use and office-automation use, such as a facsimile machine, printer or sewing machine, a power supply that activates various circuits is structured to acquire direct current (DC) from a commercial AC power supply by stepping up or down the AC current from the commercial AC power supply with a transformer. The AC current is then rectified in a half-wave or full-wave rectifier circuit, and smoothed in a smoothing circuit having a capacitor.
However, the AC current may include noise that is generated, for example, because of interference with other electric appliances, or noise that is generated internally in the electric appliance itself. Conventionally, a noise canceling circuit for eliminating noise has been provided in a power supply.
FIG. 5 illustrates a configuration of a power supply 21 including a noise canceling circuit 28 of the prior art. In the power supply 21, the primary (input) side of a transformer 23 is connected to opposite ends of an AC power supply 22, and the secondary (output) side of the transformer 23 is connected to a half-wave or full-wave rectifier circuit 24. The DC output side of the rectifier circuit 24 is connected to a smoothing circuit 25 for smoothing a pulsing current that is taken from the rectifier circuit 24. One end of the DC output side of the smoothing circuit 25 is connected to a load 26. The other end of the DC output side of the smoothing circuit 25 is connected to a grounding member, such as a machine frame of metal construction.
Capacitors 29a, 29b, 29d are connected between the AC power supply 22 and the primary side of the transformer 23. One end of the secondary side of the transformer 23 is connected between the capacitors 29a, 29b via capacitor 29c. In addition, a rocker (seesaw) switch 27 for switching the AC current on or off is disposed between one end of the AC power supply 22 and the capacitor 29a. A circuit connecting the primary side and the secondary side of the transformer 23 via a filter portion 29 composed of the capacitors 29a to 29d corresponds to the noise canceling circuit 28.
In this configuration, when the rocker switch 27 is switched on, the noise included in the AC current from the AC power supply 22 is discharged to the secondary side of the transformer 23 via the capacitors 29a to 29d. Accordingly, in the noise canceling circuit 28, the noise is eliminated from the AC current from the AC power supply 22, and stable AC current is supplied to the primary side of the transformer 23.
A problem exists, however, with the configuration of the prior art. Even with the rocker switch 27 off, the electric current flows from the AC power supply 22 through the capacitors 29a to 29d to the grounding member. The current flows over two paths. One is from the capacitor 29b to the capacitor 29c. The other one is from capacitor 29d through the capacitor 29a to the capacitor 29c because the primary side and the secondary side of the transformer 23 are connected via the filter portion 29. Therefore, when the operator touches the grounding member while the rocker switch is in the OFF position, an electrical shock proportional to a quantity of electricity accumulated in the capacitors 29a to 29d may result.
This invention provides a noise canceling circuit that overcomes the foregoing problem. To solve this problem, one embodiment of the invention has a noise canceling circuit with a filter portion including a capacitor or an inductor. This filter portion is used to connect a primary (input) side and secondary (output) side of a transformer via the filter portion in order to eliminate noise included in AC current supplied at the primary side of the transformer. The noise canceling circuit further includes a changeover device that brings the filter portion and the secondary side of the transformer into (or out of) conduction in accordance with an on-off state of a switching device disposed between the AC power supply and the noise canceling circuit. The changeover device is disposed between the filter portion and the secondary side of the transformer.
In another embodiment of the invention, the changeover device is a triac. In this configuration, when the switch device is on, the AC current from the AC power supply is stepped up or down by means of a transformer, and the AC current induced to the secondary side of the transformer is rectified in the rectified circuit. Simultaneously, the changeover device disposed between the filter portion and the secondary side of the transformer brings the filter portion and the secondary side of the transformer into conduction in accordance with an on state of the switch device. The noise included in the AC current from the AC power supply is discharged to the secondary side of the transformer via the filter portion and the changeover device. The pulsing current taken from the rectifier circuit is smoothed in the smoothing circuit, to allow DC current to be obtained.
As the noise is discharged to the secondary side of the transformer, via the filter portion and the changeover device, the noise canceling circuit according to the invention enables the noise to be eliminated from the AC current from the AC power supply. Thus, a stable AC current can be supplied to the primary side of the transformer.
When the switching device is off; the changeover device brings the filter portion and the secondary side of the transformer out of conduction in accordance with an off state of the switching device. In this case, because there is no continuity between the filter portion and the secondary side of the transformer, the current never flows to the secondary side of the transformer, and by extension never flows to the grounding member. Therefore, even if the operator touches the grounding member, there is no danger of receiving an electrical shock.
In a simple configuration, a triac is disposed between the filter portion and the secondary side of the transformer. The filter portion and the secondary side of the transformer can be brought into or out of conduction in accordance with the on-off state of the switching device, without a control device such as a microcomputer.